1. Technical Field
The disclosure relates to a method of preparing a composite optical retarder. More particularly, the disclosure relates to using a photoalignment method to prepare a composite optical retarder.
2. Description of Related Art
It is known that liquid crystal molecules have different refractive indexes at different axes to have birefringence. Hence, an optical retardation phenomenon is occurred when light passes through the liquid crystal molecules to produce phase difference. This is the optical anisotropy of liquid crystal molecules. Since the optical anisotropy will change the polarization direction of light, liquid crystal molecules can be used to control the light's transmittance to produce bright and dark effects on displays, and thus on display panels. In another aspect, since the optical retardation effect is resulted from the optical anisotropy, a liquid crystal film can be used as an optical retarder. The optical retarder with desired phase difference can be applied to a liquid crystal display to reduce light leakage and increase display contrast to achieve wide view angle effect.
No matter applied to display panels or optical retarders, liquid crystal molecules have to be aligned before they are effectively used. The earliest method to align liquid crystal molecules is forming microgroove structures having a certain direction (i.e. the alignment direction) on an alignment film to align the liquid crystal molecules to achieve alignment effect.
The conventional preparation method for such alignment films is contacting rubbing method to produce microgroove structures on the alignment films. However, the rubbing method cannot be suitably used in preparing alignment films with large area, and thus display panels with large area, since the yield is not sufficiently high and defects can easily occur. Moreover, problems of fine particles, fiber contaminations, or electrostatic effect can easily occur during the rubbing process (please see U.S. Pat. No. 6,649,231) to further influence the alignment effect of the liquid crystal molecules. In another aspect, the rubbing method cannot easily produce multi-domain alignment on an alignment film, since multiple rubbing processes are needed. Accompanying with problems of poor yield, as well as defects and particles contaminations on the surface of the alignment film, the rubbing method cannot satisfy the requirements of wide view-angle display panels via multi-domain alignment.
In order to overcome the drawbacks of the rubbing method, non-contacting alignment methods are provided. In U.S. Pat. No. 5,389,698, a photoalignment method was disclosed. A photocurable resin was irradiated by linearly polarized ultraviolet to align the resin along a preset direction. After crosslinking the resin, the resin with a fixed alignment direction can form a photoalignment film. The mechanism of aligning liquid crystal molecules by the photoalignment film is to distribute the van der Waals force of the resin molecules on the surface of the photoalignment film along the preset directions by the action of the linearly polarized ultraviolet, and then drive the liquid crystal molecules to comply with the preset direction (i.e. the alignment direction), to achieve the alignment effect (M. Schadt, JJAP, 1992). The forgoing van der Waals force distributed along the preset direction is due to the specific distribution of the functional groups or side chains of the resin molecules on the surface of the photoalignment film. Therefore, the distribution of the electron cloud or dipole moment also follows the specific distribution.
The advantage of the photoalignment method is that there is no need to rub or touch the surface of the alignment film to induce the liquid crystal molecules to align with a certain direction. Therefore, the conventional problems about particles and electrostatic effect occurred in the rubbing method can be solved. In another aspect, the photoalignment method can be applied to flexible, arc-shaped, or any other random structured substrate. The limitation of planar hard substrate required by the rubbing method can be overcome. Hence, the photoalignment method can be applied on roll-to-roll continuous process to mass produce the alignment films. In addition, the photoalignment method can also be applied to form another alignment film, with a different alignment direction, on an aligned liquid crystal film without damaging the underlying aligned liquid crystal film's surface. Thus, a composite optical retarder having multiple alignment films and liquid crystal films with different alignment directions can be prepared. Moreover, the alignment directions can be arbitrarily set by the photoalignment method. This is hard to be done by conventional rubbing method.
For meeting various requirements of various types of liquid crystal displays (LCDs), the prior arts used the photoalignment method to stack alignment films with different alignment directions and a liquid crystal layer to decrease light leakage. For example, a vertical alignment LCD needs a positive A plate and a negative C plate to compose a composite optical retarder to compensate the needed phase difference, then the liquid crystal display can reach a better contrast and wide view angle. Or, the cholesterol liquid crystal used in a bright enhancement film of a LCD needs a positive A plate and a positive C plate to compose a composite optical retarder to increase the contrast and improve the hue error problem.
Such applications all need two alignment films to prepare two optical retarders with different alignment directions. In U.S. Pat. No. 6,717,644, a composite optical retarder having two different functional layers (different alignment directions or different phase difference) is disclosed. Two alignment films are used to respectively align two liquid crystal molecular layers. However, the conventional material for the alignment films is quite expensive. Therefore, using two alignment films will increase the production cost and the thickness of the composite optical retarder. It can't meet the requirement of the thinned display panels.
Therefore, a preparation method for a composite optical retarder with a lower production cost is needed.